1. Field of the Invention
The invention generally relates to control systems for improving the operation of gyroscopic instruments and more particularly relates to a gyroscopic indexing mechanism applicable, for example, in precision gyrocompasses.
2. Description of the Prior Art
As is well understood in the prior art, there are two basic reasons for mechanically indexing a gyrocompass sensitive unit. The first provides a means for detecting case fixed drift errors of the gyroscope so that their adverse effects can be eliminated, while the second facilitates measurement of the two components of an input rate with a gyroscope having a single sensitive axis.
In the first instance, the gyroscope casing is normally indexed at 0.degree. and 180.degree. with respect to an earth-fixed index so that the sign of any measured rate is reversed. Then, by subtracting the rate measured in one position of the case from that measured with the case index at 180.degree., the resulting measure is twice the gyro input rate and is independent of any fixed gyro drift, since the latter cancels out of the picture.
In the second application of gyro indexing, the gyroscope casing is normally indexed at 0.degree. and 90.degree. with respect to an earth-fixed index so that a gyroscope sensitive element with a single input axis can measure the orthogonal components of the input rate. It is this configuration which finds application, for instance, in a land compass. The gyroscope is indexed at 0.degree. and 90.degree. about a substantially vertical axis so that the single sensitive axis can measure the two orthogonal horizontal components of earth's rate. The azimuth orientation of the gyro case is then computed in the conventional manner as the arctangent of the quotient of the two measurements.